point light strength in cycles renderer
Ok, I believe I solved the issue. Although my material's diffuse reflection was set to [1, 1, 1], for physically correct rendering the actual diffuse reflection must be divided by $\pi$. In other words, an energy conserving diffuse BRDF is defined as $f(w_i, w_o) = \frac{k_d}{\pi}$, where $k_d$ is the diffuse reflectance (note that my specular reflectance was zero so we are not concerned about the specular part).

This means that if light fall off is disabled, the actual radiance that gets reflected off the face of the cube is $\frac{10}{4\pi^2}$ (with physically correct quadratic fall off this would be $\frac{10}{4\pi^2d^2}$ where $d$ is the distance).

So in my case, this value turns about to be $0.253$ and that is what I see as pixel value in the center of the cube's face if I save the rendered image in EXR format.

As for the PNG format, cycles appears to simply multiply this number with $255$, the maximum storable value in 8-bits. This corresponds to $64$. This happens because I had disabled gamma correction. Otherwise, cycles would have gamma corrected the value before multiplying it with $255$. This produces a value of $138$ and I confirmed that by experimentation.